The introduction of intensive, high-dose, multi-agent chemotherapy has improved the event free survival (EFS) for children and young adults with T-cell acute lymphoblastic leukemia (T-ALL) from 15%-20% to 50%-85%. Despite these advances, relapsed T-ALL has a dismal prognosis with reported 3-year EFS rates of <15%. Accordingly, the primary goal in the treatment of T-ALL is to prevent relapse. Integral to this goa is appropriate risk stratification in order to determine which patients need more intensive therapy or alternative approaches for cure. Unlike B-ALL, where a combination of genetic alterations and MRD can accurately predict outcome, in T-ALL genetic abnormalities are not independently prognostic in MRD-based risk stratification. While MRD has improved risk stratification in T-ALL, the majority of relapses occur in patients classified as lower risk. Despite significant heterogeneity in genetic alterations in T-ALL, recent data from our group and others suggest that many patients with high risk T-ALL share a limited set of deregulated signaling pathways and cellular processes. Enhanced understanding of these deregulated pathways is needed in order to meet our long-term goal: To improve risk stratification and to develop novel targeted therapies for children with high risk T-ALL. AALL1231 is phase 3 Children's Oncology Group (COG)-initiated CTEP-approved clinical trial testing the hypothesis that the addition of the proteasome inhibitor bortezomib to standard T-ALL therapy will improve survival. Our group was instrumental in the development of that trial, and we will lead the clinical trial, as well as, perorm the correlative biology. We will improve understanding of the biochemical underpinnings of T-ALL by detailed mechanistic analysis of patient samples collected from AALL1231, which we will correlate with outcome data. Based on our strong preliminary data, we hypothesize that the pattern of protein cell stress activation, proteasome alterations, and signaling network activation will ascertain biologically relevant deregulated pathways that we can: (1) effectively target with novel agents; and (2) use to develop a high-risk protein expression profile that can predict outcome when combined with MRD. We will test our hypothesis with the following specific aims: (1) We will determine if changes in proteasome function or cell stress expression patterns can predict drug response and resistance in T-ALL patients treated on AALL1231; (2) We will investigate cell signaling pathways in T-ALL in order to identify and target biologically relevant abnormal activated pathways; and, (3) We will identify patients currently classified as lower risk at diagnosis that have poor outcome and need alternative therapy and use them to develop a more accurate risk allocation schema through improved MRD testing and through the development of a predictive high risk protein expression profile. We anticipate these integrative translational highly mechanistic analyses will lead to improved outcome for children with T-ALL in the near future through improved risk allocation and the development of rational biologically-based targeted therapies.